1. Field of the Invention
The present invention relates, in general, to a vibration-insensitive interferometer using a high-speed camera and a continuous phase scanning method, and, more particularly, to an interferometer for meteorological observations, which can measure the surface of a target in an environment having serious disturbances such as vibrations or air movements, using an optical interference method, with high precision and resolution and at high speed.
2. Description of the Related Art
Generally, in order to measure the shape of manufactured optical parts when the optical parts, wafers or glass products are being manufactured, a Fizeau interferometer, a point diffraction interferometer, etc. based on the principle of interference have been used. Methods of processing optical parts such as lenses or mirrors overcome the disadvantages of existing simple polishing processes, and have gradually advanced towards automation, enabling spherical or aspherical free-form surfaces having a variety of curvatures to be processed at the present time, and such automation has been widely popularized.
However, this processing technology is automated whereas measurement equipment for the technology is not yet automated, and thus a processor and a measuring device are independent of each other without there being formed therebetween a feedback-based control relationship. Further, as the size of devices increases in semiconductor wafers or display fields such as Liquid Crystal Displays (LCDs) and Plasma Display Panels (PDPs), a measuring device capable of measuring a wide area at one time is required, but, in fact, a suitable method complying with this requirement has not yet been provided.
The most significant cause of these problems is that an optical interferometer used as a measuring device is greatly susceptible to changes in the environment, in particular, to vibrations. Due thereto, measurement results are inadequate, and consequently the optical interferometer cannot be applied to a production process for producing optical parts where there are serious vibrations.
In order to solve these problems, research into interferometers less susceptible to vibrations has been conducted using a variety of methods.
One of these methods is a method of directly measuring the variation in an interference fringe attributable to vibrations using a sensor, and feeding the interference fringe variations back to the driving of a reference reflective surface, thus stabilizing the interference fringe (refer to T. Yoshino et al., Opt. Lett., 23, p.1576). However, in this method, since vibration frequency that can be controlled is limited according to the bandwidth of the sensor and an actuator, it is impossible to prevent interference fringes from being averaged due to high-frequency vibrations which are above the vibration frequency.
Another method, that is, spatial phase shifting, is known to be less susceptible to vibrations because it enables the analysis of interference fringes in real time (refer to R. A. Smythe et al., Opt. Eng., 23, p.361). FIG. 1 is a diagram schematically showing a spatial phase-shifting interferometer. However, this scheme is also disadvantageous in that the vibration frequency that can be controlled is limited according to the bandwidth of a camera used for measurement, and repeatability between the results of successive measurements greatly decreases due to the influences of different types of vibrations at the time of repeatedly performing the measuring, thus making it impossible to obtain reliable measurement values.
A further method, that is, a common path interferometer, is advantageous in that, since similar vibration components exist both on a reference wavefront and on a measurement wavefront, interference fringes can be basically stabilized.
Further, an interferometer using a diffraction grating and a pinhole may be a solution capable of taking the advantages of all of the above-described interferometers and overcoming the disadvantages thereof because it can simultaneously obtain three spatial phase-shifted interference fringes while supporting a common path (refer to Osuk Y. Kwon et al., Opt. Lett., 12, p.855). However, it is very difficult to actually implement such an interferometer, and it is also difficult to obtain precise measurement results because part of a distorted measurement wavefront is used as a reference wavefront generated by the pinhole.
In this way, most conventional technologies intended to obtain vibration-insensitive characteristics in shape measurement fields using optical interferometers have focused on the stabilization of interference fringes to eliminate phase vibration characteristics themselves appearing on the interference fringes. For this operation, a feedback method based on the direct detection of light intensity versus phase, a common path structure for offsetting the influences of vibrations, and a one-shot measurement using a spatial phase-shifting device or a spatial heterodyning method have been applied.
However, those methods are disadvantageous in that they require complicated optical parts for the elimination of vibration influences and phase shifting from the standpoint of a system structure, have limitations in spatial resolution, and have difficulty in individually coping with the influences of vibrations appearing differently for respective pixels.